Add-On Dev: Operator Window - Open/Close events?
I think I can safely say these are reliable, for a couple reasons:


They are reliable for modal operator classes, and there is little difference between a modal operator class and a non-modal operator class, except for a specific function being defined and returning certain values from specific functions. It makes sense that the constructor/destructor can be relied on for all operator classes.
See https://docs.blender.org/api/current/info_overview.html (thanks to Brockmann for updated version link) .. If you look under the section titled class mix-in example, it states the following:

"Notice these classes don’t define an __init__(self) function. While __init__() and __del__() will be called if defined, the class instances lifetime only spans the execution."


If anyone can confirm or has any reason to think otherwise, please let me know. Thanks!